Breast Cancer & Dark Skies

I'm always a bit skeptical of correlations that fit with what I want, and I'm skeptical of this one as well--but there is at least a plausible causal connection here, and because I am an amateur astronomer, I'm not going to spend a long time looking this gift horse in the mouth. (Several years ago, the Idaho Statesman published a piece by me about the importance of dark skies.) From the February 20, 2008 Washington Post:

Women who live in neighborhoods with large amounts of nighttime illumination are more likely to get breast cancer than those who live in areas where nocturnal darkness prevails, according to an unusual study that overlaid satellite images of Earth onto cancer registries.
The finding adds credence to the hypothesis that exposure to too much light at night can raise the risk of breast cancer by interfering with the brain's production of a tumor-suppressing hormone.
"By no means are we saying that light at night is the only or the major risk factor for breast cancer," said Itai Kloog, of the University of Haifa in Israel, who led the new work. "But we found a clear and strong correlation that should be taken into consideration."
Scientists have known for years that rats raised in cages where lights are left on for much of the night have higher cancer rates than those allowed to sleep in darkness. And epidemiological studies of nurses, flight attendants and others who work at night have found breast cancer rates 60 percent above normal, even when other factors such as differences in diet are accounted for.
On the basis of such studies, an arm of the World Health Organization announced in December its decision to classify shift work as a "probable carcinogen." That put the night shift in the same health-risk category as exposure to such toxic chemicals as trichloroethylene, vinyl chloride and polychlorinated biphenyls (PCBs).

...

The team then overlaid that map with local statistics on cases of breast cancer and, for comparison, lung cancer, which is caused mostly by smoking and so would not be expected to be linked to light.
After using neighborhood data to correct for other factors that can affect cancer rates, including wealth, ethnicity and the average number of children in families living in those localities, the researchers found no link between night lighting and lung cancer, they report in this week's online issue of the journal Chronobiology International.
But the researchers found the breast cancer rate in localities with average night lighting to be 37 percent higher than in communities with the lowest amount of light; and they noted that the rate was higher by an additional 27 percent in areas with the highest amount of light.

That matter of the compact fluorescents is especially interesting and worrisome.

Like I said, there are all sorts of other possible reasons for this correlation. Perhaps women that live in big cities, with lots of night illumination, and big city values, are more likely to smoke.

There have been some studies that suggest a link between abortion and cancer, although the National Cancer Institute says that the most recent and best done studies do not show the link. Here's a study from the New England Journal of Medicine which "370,715 induced abortions among 280,965 women" and "10,246 women with breast cancer" from Danish records, and found no correlation.

This 1994 study published by the Journal of the National Cancer Institute, on the other hand, did find a correlation:

Highest risks were observed when the abortion was done at ages younger than 18 years—particularly if it took place after 8 weeks' gestation—or at 30 years of age or older. No increased risk of breast cancer was associated with a spontaneous abortion (RR = 0.9; 95% CI = 0.7-1.2).

Similarly, this 1996 article from Journal of Epidemiology and Community Health reviewed existing studies, and concluded:

Although the increase in risk was relatively low, the high incidence of both breast cancer and induced abortion suggest a substantial impact of thousands of excess cases per year currently, and a potentially much greater impact in the next century, as the first cohort of women exposed to legal induced abortion continues to age.

This 2004 article in Cancer Causes and Control found no correlation, but I notice that it looked at both induced and spontaneous abortions--and that Journal of the National Cancer Institute study that found a correlation with induced abortions didn't find it with spontaneous abortions. It is conceivable that the correlation is weak enough that combining spontaneous abortions might wash out the results of induced abortions.

I don't find a link between abortion and breast cancer implausible (there's a lot of hormonal consequences to pregnancy, and an abortion would certainly affect hormonal activity), but considering the number of variables that are correlated with abortion, I can see how even if such a connection exists, it would be very difficult to definitively prove or disprove.